In order to prevent counterfeiting by means of scanning and copying, optical anti-counterfeiting technology is widely used in various high-security or high value-added printed matters such as banknotes, cards and product packaging, and has achieved very good effects.
Patent documents CN101563640, CN101443692, CN101120139, CN101346244, U.S. Pat. Nos. 5,712,731, 0,034,082, 4,765,656, 4,892,336, CN1271106 and CN1552589 disclose the same kind of anti-counterfeiting elements with a micro-lens array and a micro image-text array on two surfaces of a substrate respectively, wherein the micro image-text array is located near the focal plane of the micro-lens array and reproduces images with a certain depth of field or dynamic effects through the Moiré magnification of the micro image-text array by the micro-lens array.
To ensure that Moiré magnified patterns are easy to identify under different ambient light conditions, the micro image-text array and its background need to have sufficient contrast in color or brightness, that is, the micro image-text array needs to be colorized. Given that the required structure of micro image is very fine (about a few microns), such an accuracy grade is unreachable by utilizing a general printing technology which has a printing stroke of 20 microns or more.
There are two colorizing methods at present. One method is disclosed in patent documents (e.g., CN1906547A) as follows: forming grooves with a certain depth in the micro image-text region, filling the grooves with stain material by means of the blade coating processes, and the excessive materials which are located beyond the micro image-text region shall be substantially removed. In order to obtain a favorable colorizing effect, this method is significantly restrictive on the width of lines and depth of grooves of the micro image as well as matching relationship therebetween.
The other method is applied based on the micro-nano structure, for instance, patent document US20030179364 discloses a solution of performing a black colorization of the micro image by means of an optical absorbing structure with a high width-to-depth ratio, and the patent document US20100307705A1 discloses a solution of performing a colorization of the micro image by filling with nanoparticles or benched metallic nanostructure.
However, the above two coloring methods have defects in different aspects: US20030179364 can hardly reach a colorful tinting, and the benched structure and the manner of planar covering the structure by the coating utilized in US20100307705A1 can hardly be applied in the production.